From a regional perspective, the late thirteenth-century aggregation of village populations into large towns in the northern Southwest appears to be a brief and dramatic episode of social reorganization. That it is apparent across such a range of cultural and ecological circumstances suggests that a regional perspective will be needed to understand why it occurred. However, if we are to understand how it occurred—the social processes involved in the aggregation of populations into large towns—a high resolution, long-term view of the settlement history of particular localities provides a necessary complement to the regional view. We present a detailed examination of the development of one town in the American Southwest, the large, prehistoric Zuni town of Heshot uła. Without the long-term demographic reconstruction made possible by a fine-grained seriation and full-coverage survey, we would have seen this key transition as much more rapid and much less closely tied to the local situation than it now appears to be. Although this pueblo was built about A.D. 1275, we argue that its appearance was, at once, the culmination of demographic processes operating over at least 150 years, and the outcome of two more rapid, qualitative organizational transformations separated by a century.